diff --git a/e2e/config/librechat.e2e.yaml b/e2e/config/librechat.e2e.yaml
index 7da6aa4b10..6b8a375d41 100644
--- a/e2e/config/librechat.e2e.yaml
+++ b/e2e/config/librechat.e2e.yaml
@@ -4,6 +4,16 @@
 version: 1.3.11
 cache: true
 
+# Unified file upload routing. Spreadsheet/csv files resolve to llmDeliveryPath
+# 'none' (kept out of LLM context + provider delivery; reachable only via tools
+# like code interpreter). Exercised by e2e/specs/mock/unified-upload.spec.ts.
+fileConfig:
+  defaultLLMDeliveryPath:
+    overrides:
+      'text/csv': 'none'
+      'text/plain': 'none'
+      'application/vnd.openxmlformats-officedocument.spreadsheetml.sheet': 'none'
+
 endpoints:
   custom:
     - name: 'Mock Provider A'
diff --git a/e2e/specs/mock/unified-upload.spec.ts b/e2e/specs/mock/unified-upload.spec.ts
new file mode 100644
index 0000000000..3746cf19fb
--- /dev/null
+++ b/e2e/specs/mock/unified-upload.spec.ts
@@ -0,0 +1,83 @@
+import { expect, test } from '@playwright/test';
+import type { Page } from '@playwright/test';
+import {
+  MOCK_ENDPOINTS,
+  NEW_CHAT_PATH,
+  fetchJson,
+  getAccessToken,
+  selectMockEndpoint,
+} from './helpers';
+
+/**
+ * Unified file upload — per-mime-type delivery routing (PR #12626).
+ *
+ * What this proves end-to-end (real backend + DB), and what it deliberately can't:
+ * - The composer renders ONE attach button (unified mode), not the legacy 3-way
+ *   dropdown — `legacyFileUploadUX` is unset in e2e/config/librechat.e2e.yaml.
+ * - A spreadsheet/csv upload routes through `processAgentFileUpload` and persists
+ *   `llmDeliveryPath: 'none'` per the configured `defaultLLMDeliveryPath` override,
+ *   i.e. it is NOT delivered to the LLM as text/provider — reachable only by tools.
+ * - The mock harness has no code-execution environment, so the "available to the
+ *   code interpreter at tool-execute time" half is covered by jest
+ *   (`packages/api/src/agents/resources.test.ts`), not here.
+ *
+ * `.xlsx` follows the identical code path (same `defaultLLMDeliveryPath` override,
+ * same standard-storage branch); csv is used here so the uploaded bytes match the
+ * declared mime type without synthesizing a binary spreadsheet.
+ */
+
+const uniqueName = (p: string) => `${p}-${Date.now()}-${Math.floor(Math.random() * 1e4)}`;
+
+type UploadedFile = { filename?: string; type?: string; llmDeliveryPath?: string };
+
+const isFilesUpload = (url: string, method: string) =>
+  method === 'POST' && /\/api\/files(?:\?|$)/.test(new URL(url).pathname);
+
+async function uploadViaUnifiedButton(page: Page, fileName: string) {
+  const csv = 'name,score\nalice,1\nbob,2\n';
+  const uploadResponse = page.waitForResponse((r) => isFilesUpload(r.url(), r.request().method()), {
+    timeout: 30000,
+  });
+  const [fileChooser] = await Promise.all([
+    page.waitForEvent('filechooser'),
+    page.locator('#attach-file-button').click(),
+  ]);
+  await fileChooser.setFiles({
+    name: fileName,
+    mimeType: 'text/csv',
+    buffer: Buffer.from(csv, 'utf8'),
+  });
+  return uploadResponse;
+}
+
+test.describe('unified file upload', () => {
+  test('single attach button routes a csv to llmDeliveryPath "none"', async ({ page }) => {
+    test.setTimeout(120000);
+    await page.goto(NEW_CHAT_PATH, { timeout: 10000 });
+
+    // Default model needs a real key; pick a mock endpoint so the composer is live.
+    await selectMockEndpoint(page, MOCK_ENDPOINTS[0]);
+
+    // Unified mode: one attach button, and the legacy multi-option dropdown trigger
+    // is not rendered at all.
+    await expect(page.locator('#attach-file-button')).toBeVisible({ timeout: 15000 });
+    await expect(page.locator('#attach-file-menu-button')).toHaveCount(0);
+
+    // Upload-time routing: the configured override (csv -> none) must persist, so
+    // the file is kept out of LLM delivery and left for tools (code interpreter).
+    const fileName = `${uniqueName('data')}.csv`;
+    const response = await uploadViaUnifiedButton(page, fileName);
+    expect(response.ok()).toBeTruthy();
+
+    const uploaded = (await response.json()) as UploadedFile;
+    expect(uploaded.filename).toBe(fileName);
+    expect(uploaded.llmDeliveryPath).toBe('none');
+
+    // Persistence: the file is queryable from the backend with the same routing.
+    const token = await getAccessToken(page);
+    const files = await fetchJson<UploadedFile[]>(page, '/api/files', token);
+    const persisted = files.find((f) => f.filename === fileName);
+    expect(persisted, `uploaded file "${fileName}" should persist`).toBeTruthy();
+    expect(persisted?.llmDeliveryPath).toBe('none');
+  });
+});